Standart HX290U, Service Manual

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3
2. THEORY OF OPERATIONS
Note : Refer to the block diagrams (figure 2-1 through 2-8) in the text for the operations of the circuits.
2.1 PLL Block
The PLL block comprises a VCO, TCXO, PLL IC, and PLL loop filter.
The VCO circuit generates the transmission signal and first local signal directly.
2.1.1 Programmable Divider
The input oscillation signal is frequency divided by the
prescaler using a determined division ratio (1/64 or 1/65).
After this, the oscillation signal is input to a programmable
divider built into the PLL IC. Based on the data from
microprocessor QL02, the programmable divider
frequency divides the oscillation signal from the VCO to
1/N to generate a comparison frequency (fp) of 5 kHz or
6.25 kHz.
2.1.2 Reference Divider
The reference divider is a circuit that generates a
reference frequency (fr) of 5 kHz or 6.25 kHz based on
data from microprocessor QL02. The 12.8 MHz reference
oscillation signal from the TCXO passes through pin 1 of
PLL IC QP02 and is input to the reference divider built
into the PLL IC.
The 12.8 MHz reference oscillation signal is frequency
divided to 1/2,560 (1/2,048) to generate a reference
frequency of 5(6.25) kHz.
2.1.3 Phase Comparator
The phase comparator compares the comparison
frequency (fp) and the reference frequency (fr) to
determine the phase difference.
2.1.4 Charge Pump
The charge pump circuit charges and discharges the
electrical charge accumulated in the PLL loop filter.
2.1.5 PLL loop Filter
The PLL loop filter CR integrates the level signal (square
wave) output from the charge pump, converting it into a
DC voltage.
1/N
PLL loop filter
Audio signal
TCXO
8
Charge pump
Phase comparator
5
1
14
9 10 11
QP04
DTC114YE
(fr)
(fp)
PLL IC
Unlock
5 kHz
(6.25 kHz)
5 kHz
(6.25 kHz)
QP02 - MB15E03
QL02
81
82
77
28
Strobe
Data
Clock
1/2560
(1/2048)
Reference divider
Programmable divider
Prescaler
Microprocessor
Unlock switch
12.8 MHz
2SC5005
QV07
VCO buffer amplifier
DAN235E
QP09
TX/RX switch
Transmission signal (TX)
First local signal (RX)
2SC5005
QP08
(0 dB amp)
Buffer amplifier
Approx. 300 mV
XP01
2SK1875(V)
QV06
VCO RX
2SK1875(V)
QV04
VCO TX
VCO
2SC4726
QP10
Buffer amplifier
Figure 2-1 PLL block diagram